Focimeters

[jennyj]

Hi guys!!
i was just wondering if anyone can help me...
what are the advantages and disadvantages of replacing a +20D standard lens with a +10D lens?

[Darryl Meister]

I am just guessing here, since I would have to sit down and really work through the principles involved for a more confident answer, but I would say that a weaker standard lens will increase the length of the focimeter, reduce the range of powers you can measure, but give you more precision in your measurements and perhaps achieve some other benefits related to aberrations, stops, and that sort of thing.

You can't measure powers any higher than your standard lens, at least without using an auxiliary lens. Further, a weaker standard lens will require more movement to cover the same range of power, which is why you could get more precision in your power measurement.

Best regards,
Darryl

[David Wilson]

Just adding to what Darryl has said, you would also need to put the standard lens in a different position. The lens rest aperture should be at the second principal focus of the standard lens.


Regards
David

[Darryl Meister]

Just adding to what Darryl has said, you would also need to put the standard lens in a different position.

To be sure, the instrument would have to be redesigned/re-calibrated.

Best regards,
Darryl